The present invention relates to a drivetrain for an all-wheel-drive vehicle which has a first driven axle and a second driven axle, having a drive unit which is connected to a multi-stage transmission which has a multiplicity of gear stages for generating different transmission ratios, with a transmission output shaft of the multi-stage transmission being connected to the first driven axle, with the drive unit also being connected to an activating arrangement, the output element of which is connected to the second driven axle, such that drive power can be supplied to the first axle permanently via the multi-stage transmission and drive power can be supplied to the second axle on demand via the activating arrangement.
The present invention also relates to a method for controlling a drivetrain which has a drive unit which is connected firstly via a transmission to a first axle and secondly via an activating arrangement to a second axle.
In recent years, interest in all-wheel-drive motor vehicles has risen. Here, all-wheel drive is no longer used exclusively in off-road vehicles. The improved traction of all-wheel-drive drivetrains is also advantageous in normal road-going vehicles.
In general, in all-wheel-drive drivetrains, a distinction is made between clutch-controlled and differential-controlled systems. A differential-controlled system has a central longitudinal differential which receives drive power from a drive unit via a multi-stage transmission and distributes said power permanently between the two driven axles. In clutch-controlled all-wheel-drive drivetrains, it is generally the case that one axle is permanently driven and the second axle is activated on demand (“hang-on” systems).
In off-road vehicles, the second axle is often activated manually. In road-going vehicles, this generally takes place by means of an automatically closing clutch.
Here, use was made in the past of passive systems, such as for example a Visco clutch or a Haldex clutch. Said known systems often operate such that the clutch is closed in reaction to a rotational speed difference between the permanently driven axle and the non-permanently driven axle.
The introduction of ABS and ESB also places increased demands on all-wheel-drive systems. Active systems have therefore been developed which can be opened or closed at any time regardless of rotational speed differences. The so-called disturbance torques of passive systems are prevented in this way. In the active systems, use is often made of friction clutches. On account of the good controllability and the good wear behaviour, said friction clutches are often embodied as multiplate clutches, in particular as wet-running multiplate clutches.
All-wheel drive also has advantages, in particular with regard to the acceleration performance which can be obtained, in sporty motor vehicles in which a high power-to-weight ratio is important.
Conventional all-wheel-drive drivetrains, however, are afflicted with a relatively high overall weight.
Document WO 2006/100585 A1 discloses a drivetrain for a motor vehicle which is suitable for sporty vehicles. Here, a drive unit in the form of an internal combustion engine is connected via a multi-stage transmission having five, six or more gear stages to a first driven axle (the rear axle).
Furthermore, the output of the drive unit is connected to an activating (“hang-on”) arrangement in the form of a single friction clutch whose output element is connected to the second driven axle. Here, the activating arrangement may, as one option, be connected to an output element of a starting and separating clutch which is arranged between the drive unit and the multi-stage transmission. It is alternatively also possible for the activating arrangement to be connected directly to an output shaft of the drive unit (for example the crankshaft of an internal combustion engine). This permits, for example, a layout in which the starting and separating clutch and also the multi-stage transmission are connected, on a first axial side, to the output shaft of the drive unit. In this embodiment, the activating arrangement is connected, on the other axial side, to the output shaft of the drive unit.
Here, the friction clutch which is used in the activating arrangement of the drivetrain of WO 2006/100585 A1 is connected via a transmission ratio stage to the second driven axle, in such a way that the rotational speed of the second driven axle corresponds to a certain gear stage of the multi-stage transmission, in particular the third gear stage, when the friction clutch of the activating arrangement is closed. In this way, all-wheel drive is possible only in said gear stage and in the lower gear stages (that is to say for example the second and first gear stages).
Furthermore, the output element of the activating arrangement of the drivetrain of WO 2006/100585 A1 is connected to a mechanical differential of the second driven axle. An input member of the activating arrangement may be connected via a spur gear set arrangement to the drive unit. Alternatively, an output element of the activating arrangement is connected via a spur gear set arrangement to an input element of the second driven axle. The transmission ratio of the spur gear set arrangement and the transmission ratio of the mechanical differential form the overall transmission ratio, which determines the individual gear stage to which the activating arrangement is assigned.
While the mechanical differential of the first driven axle may be embodied as a limited-slip differential, the differential of the second driven axle may be embodied as an “open” differential. It is also proposed in document WO 2006/100585 A1 to electronically control the percentage of the drive torque which is allocated to the second driven axle according to dynamic parameters of the motor vehicle.